1. Field of the Invention
This application is related to precision cleaning systems and, more particularly, apparatus for cleaning parts with supercritical fluids, with or without supplemental cleaning techniques.
2. Description of the Prior Art
Today's manufacturing and assembly industries require parts which have a high degree of cleanliness. These requirements have led to the development of an independent area of technology known as "precision cleaning". Precision cleaning may be defined as cleaning a given part to a degree that the level of foreign substances on the part meets a repeatably measurable standard. For example, parts which are to be chrome plated must be cleaned to a contaminant level of 20 micrograms per square centimeter or less. Disc drive components for computers must be cleaned to a level less than 5 micrograms per square centimeter, and wafers utilized in the electronics industry must be cleaned to a level less than 1 microgram per square centimeter. The various contaminants removed by precision cleaning include dissolvables, such as cutting fluid, particulates, such as diamond dust, and ionic bindings. Applications for precision cleaning include the manufacture of pens, razors and computer chips as well as various electronics industry applications.
The problem with presently available precision cleaning systems is that they use chlorofluorocarbons (CFCs) which are considered to destroy the earth's ozone layer. A system which utilizes CFCs is disclosed in U.S. Pat. No. 4,443,269 to Capella et al. ("Capella"). Capella discloses a decontamination method for radioactive tools utilizing a high pressure spray gun for spraying the contaminated tolls with freon. The general solution is to utilize more benign cleaning solvents, such as carbon dioxide. Carbon dioxide is particularly advantageous because it is a nonpolar solvent so that cosolvents may be added for a high degree of selectivity. It has been found that the cleaning capability of solvents such as carbon dioxide is enhanced when the solvent is raised to supercritical temperatures and pressures, or when supplemental cleaning techniques are utilized, such as pressure pulsing and sonic treatment.
The general concept of cleaning with supercritical fluids is known in the art. U.S. Pat. No. 5,013,366 to Jackson et al. ("Jackson") discloses a cleaning process using phase shifting of dense phase gases. The solvent is shifted from its critical state to the liquid state and back by temperature adjustment while the solvent is in contact with the part to be cleaned. The cleaning apparatus utilized in Jackson is shown in FIG. 6. However, Jackson discloses no internal filtration for the cleaning fluid so that cleaning fluid which has removed contaminants from the part may redeposit the same on the part during circulation through the vessel.
Other cleaning systems utilizing circulated fluids, such as air, are disclosed in U.S. Pat. Nos. 4,936,922; 4,832,753; 4,844,743; 4,576,792; and 4,290,821. However, none of these systems is directed to supercritical cleaning. An apparatus for treating a workpiece at elevated temperatures and pressures is disclosed in U.S. Pat. No. 4,151,400 to Smith, Jr. et al. Additionally, an electric oven having an internal circulation fan and heating elements is disclosed in U.S. Pat. No. 1,986,088 to Wild. Neither of these patents is directed to cleaning.
Finally, regarding cleaning fluid regeneration, Jackson schematically discloses in FIG. 4 a separator 28 in communication with the outlet on the cleaning vessel for recycling cleaning fluid. More detailed disclosures for devices used to separate liquid and other substances from gases may be seen in U.S. Pat. Nos. 4,879,004; 4,657,487; 4,441,871; 3,997,303; and 3,063,259. However, none of these patents is directed to a filter or separator for use in regenerating cleaning fluids for precision cleaning.
The apparatus used for precision cleaning is critical to success of the system. The apparatus must afford a high degree of temperature and pressure control and must be resistant to the high pressures required to achieve supercritical states. The apparatus must also be adaptable to remove particulates to varying degrees, especially when the apparatus is used in connection with parts testing. Importantly, the apparatus must be adapted to continuously circulate cleaning fluid in a closed system, yet avoid reapplication of contaminants onto the parts. Finally, the apparatus must provide uniform temperature and pressure throughout the cleaning vessel, while simultaneously maintaining different temperatures and pressures in other components of the overall system. It is an advantage, according to this invention, to satisfy all of the above-stated requirements.